This study is an old but ongoing study evaluating the efficacy of recombinant alpha interferon in patients with hairy cell leukemia. Fifty-six patients have been admitted to this study and of these 43 continue on therapy. The overall initial response rate was 92 percent of 54 patients evaluable for response. There have been 2 complete responses, 33 partial responses, 15 minor responses, and 4 patients did not respond. Two patients are not evaluable for hematologic response. One received only a single dose of interferon prior to the discovery of a malignant glioma, and the second patient is not evaluable for hematologic response because she was treated for painful bony involvement proven to be due to hairy cells. One patient who did not respond at 3 million units daily was dose escalated to 6 million units daily and subsequently had a minor response which is ongoing. Seven patients who responded initially ultimately had disease progression. Six of these were treated with higher doses of interferon and 3 have responded. The response in these 3 patients however has been limited and transient. Forty-three responding patients continue to respond. Toxicity has been minimal and acceptable. The most common side effect has been minor fatigue which in no way has been limiting.